Mental and Physical Abuse

I'm a 22 year old girl.My father is a police constable. My mother is a central government employee. I used to have a younger brother earlier but unfortunately he expired in 2012. Even before my brother died my father had an affair with one lady who was our tenant. Since then my father is abusing my mother physically as well as mentally. He is not looking after us. My mother alone is striving hard and looking after the family. Me, my father, my mother and my mother's mother will stay in our house. Even though he is not giving money to run the house my mother did not put up a fight. She kept calm. Now my father is going beyond limits. He is breaking all the valuable things in the house. And physically abusing. There is no peace in the house. I'm unemployed right now. I'm preparing for civil services. And there is no peace of min at home. Everyday he is creating some issue without any reason and is abusing us verbally. Please give us a lead as we are afraid to take a step. He is already threatening us that nobody can touch him as he is a police.